Ultra-low frequency (ULF) electromagnetic (EM) waves are the preferred transmission mechanism for wireless subterranean telemetry applications due to the ULF wave's ability to propagate long distances through the Earth's strata. In a typical subterranean telemetry application, the desired telemetry information is digitally encoded into data packets and sent as modulated “bursts” of ULF carrier waves. Transmission of the carrier waves is physically facilitated by injecting a modulated current into the Earth media using a power amplifier to create a time-varying voltage potential between two transmit electrodes coupled to the Earth media. The electrodes are spaced such that the induced current traverses a section of the Earth media creating associated electric and magnetic field energy which radiates as time-varying wave fronts through the Earth media.
According to a conventional EM telemetry system, a lower portion of drill string is electrically isolated from the upper portion, permitting the electrically-isolated lower portion to act as an antenna to transmit or receive ULF carrier waves to or from the surface through the Earth's strata. Transmission and reception by the antenna is enabled by circuitry within a transceiver located in the lower drill string portion below the point of electrical isolation. The transceiver is conventionally deployed in an antenna sub located just below the point of electrical isolation. In receive mode, the transceiver is connected to the lower drill string portion acting as an antenna that is electrically isolated from the surface. The transceiver may thus receive EM waves propagated from the surface through the Earth's strata. In transmit mode, the transceiver's tendency is to want to transmit using the entire drill string as an antenna. However, EM waves propagated by the transceiver are forced to “jump” the point of electrical isolation by passing through the surrounding Earth media. In so doing, the EM waves are thus forced to propagate through the Earth's media, where they may be received by the surface antennae. The EM system may therefore enable tools on the drill string to intercommunicate with the surface via encoded data packets modulated onto the transceived carrier waves.
In order for the lower drill string portion to efficiently function as an antenna, the lower portion should be electrically isolated from the upper portion as completely as possible. Any loss in complete electrical isolation will cause the lower drill string to start to lose its character as an antenna, reducing the effectiveness of the EM system in communicating via the Earth's strata. This need for as complete an electrical isolation as possible is magnified in view of the “reality” of the high impedance of the Earth's strata through which the carrier waves must pass in normal operational mode. In order to encourage robust wave propagation through the Earth's strata (and deter wave propagation losses to ground via the upper portion of the drill string), the impedance of the electrical isolation must be correspondingly even higher. It will be appreciated that complete electrical isolation is rarely achievable in practice. Most operational isolations will be “lossy” to some degree. A goal of electrical isolation of the drill string in EM telemetry is thus to reduce “lossiness” to as close to “no losses” as possible.
A further “reality” is that the EM waves transmitted by the transceiver on the drill string are likely to be weak in comparison to their counterparts transmitted from the surface because local power available to a transceiver on a tool string is limited. Thus, any wave propagation loss via poor isolation between upper and lower portions of the drill string is likely to cause a magnified reduction in effectiveness of the tool string transceiver's transmissions, as compared to surface transmissions.
At and around the desired point of isolation, the drill string often comprises an operational downhole tool structure deployed inside a hollow cylindrical outer collar. The collar generally refers to a string of concatenated hollow tubulars made from non-magnetic material, usually stainless steel. In this type of deployment, it is often advantageous to make separate but cooperating electrical breaks in both the tooling and in the collar itself in order to achieve overall electrical isolation of the entire drill string.
This electrical isolation of the upper and lower portions of the drill string is frequently enabled by placement of “gap sub” technology in the drill string at the point at which isolation is desired. The gap sub technology provides isolating structure to prevent, as completely as possible, any electrical conductivity through the drill string between the portions of the drill string above and below the gap sub technology.
This disclosure uses the term “gap sub technology” in the previous paragraph because in alternative deployments, the electrical isolation of the upper and lower portions of the drill string may be achieved using differing arrangements. For example, electrical isolation may be enabled by deploying, in one or more locations on the drill string, a single integrated electrical break which is integrated and continuous across both the tubular drill collar and the tooling within the drill collar. In other arrangements, as noted above, electrical isolation may be enabled via separate but cooperating electrical breaks: one (or more) electrical break(s) on the tubular drill collar, plus one (or more) separate electrical break(s) within the tooling structure deployed inside the collar. This disclosure pertains to the latter arrangement, in which the electrical isolation of the internal structure is separate from the electrical isolation of the drill collar itself.
On the collar itself, a “gap sub” is provided, comprising a hollow tubular inserted in the concatenation of hollow tubulars that comprise the collar. The concatenated connections of the collar are conventionally pin and box threaded connections, and the collar itself is conventionally a non-magnetic material (usually stainless steel). The gap sub is conventionally a non-magnetic tubular with pin and box connections at either end, configured to be inserted at a desired position in a concatenated string of similarly-connected non-magnetic drill collar tubulars. The collar itself is a portion of the overall drill string. Functionally, therefore, the gap sub electrically isolates the portions of the drill collar (and therefore, by extension, the entire drill string) above and below the gap sub.
Similarly, inside the collar, an “internal gap” is used for electrical isolation of the internal tooling structure. It is usually positioned just above the transceiver tooling. The internal gap electrically isolates the drill collar internals below the internal gap from the drill collar internals above the internal gap. Advantageously, the internal gap is also positioned as close to the external gap sub as is feasible, in order not to separate the internal gap and external gap too far within the drill string. When internal and external gaps are separated, the quality of the “jump” of EM transmissions across the gap and into the surrounding formation may be compromised.
The prior art describes various gap subs and internal gaps. For example, threaded isolation joints with ceramic-coated threads can be used to electrically isolate the drill collar. While serviceable, the durability and electrical performance of these types of isolation joints, especially in harsh environments, can be improved upon.
This disclosure is directed to an improved electrical isolation structure that provides excellent (almost complete) electrical isolation of the drill string above and below its location. Embodiments of this structure have demonstrated excellent performance in operating conditions historically known to cause the isolating structure of prior art gaps to break down or fail (e.g., high-vibration environments). These failures can cause unacceptable loss of isolation, and corresponding loss in EM telemetry, during live drilling operations. While originally conceived for electrical isolation of the drill collar (i.e., via an external “gap sub”), additional embodiments of the inventive content disclosed here have proved themselves also suitable for electrical isolation of the internal portions of the drill string (i.e., as an “internal gap”).